TiO2–LiF composite coating for improving NCM622 cathode cycling stability: one-step construction

The Ni-rich NCM622 is a promising cathode material for future high energy lithium ion batteries, but unstable electrochemical performance of NCM622 hinder its large scale commercial application. The cycling peformance of nickel-rich LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode materials can be improved by surface coating. Here, a one-step approach based on TiF4 is used to successfully manufacture modified NCM622 cathode materials with a TiO2–LiF coating. The TiO2–LiF coated NCM622 preserves 79.7% capacity retention which is higher than the pure NCM622 (68.9%) at 1C after 200 cycles within 2.7–4.3 V. This material serves as the cathode for lithium-ion batteries (LIBs). The uniform TiO2–LiF coating layer can alleviate structural degradation brought on by unfavorable side reactions with the electrolyte has been validated. TiO2–LiF coated on NCM622 cathode materials can be modified easily by one-step approach.


Introduction
2][3][4] The cycling performances and safety of LIBs are signicantly inuenced by cathode materials. 5,6To fulll the increased energy demands, it is imperative to research and develop high capacity, superior stability and low-cost cathode materials.Nickel-rich LiNi x Co y Mn z O 2 (NCM, x $ 0.6) cathode materials have drawn a lot of interest due to its high specic capacity and low cost. 7Among them, the LiNi 0.6 Co 0.2 -Mn 0.2 O 2 (NCM622) cathode materials offer higher reversible capacity and superior cycling stability owing to its low Ni concentration.Therefore, NCM622 cathode material is a promising candidate for high-energyLIBs. 8,9aising the cut-off voltage can increase the discharge capacity of NCM622 cathode materials.However, the irreversible phase transition of NCM622 at high charging voltage results in the destruction of the surface structure of NCM622. 10,11Additionally, unintentionally generated side reactions between NCM622 and the electrolyte, such as the development of cathode electrolyte interphase (CEI) and the dissolution of transition metal ions, may reduce the active components and reversible capacity.Surface lithium residues on NCM622 like LiOH and Li 2 CO 3 have an impact on the increased impedance and rapid capacity fading . 12Higher Ni content of NCM622 causes oxygen evolution and surface degradation at higher voltage, which also hastens the decline in cycling performance. 135][16][17] From the standpoint of scalability, repeatability, and exibility of the material synthesis, rational composition design using elemental doping is considered to be the most practical way. 18,191][22] Surface coating is thought to be an efficient way to reduce side reactions at the CEI.4][25][26][27][28][29][30][31][32][33] To achieve a compromise between physical shielding and adequate charge transfer, metal oxides need to control coating thickness.Since the growth mechanisms of various materials in liquid are quite complex, not to mention the chemical reaction between the NCM622 and solvents, it is difficult to produce an evenly distributed and continuous layer with precise thickness using wet-chemistry processes like solgel and precipitation methods. 34,35With the exception of cost restrictions, gas phase technologies like chemical vapor a Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen, Fujian 361005, People's Republic of China.E-mail: hangguo@xmu.edu.cn

RSC Advances
PAPER deposition (CVD) 36,37 and atomic layer deposition (ALD) [38][39][40] are preferable options.Contrarily, despite its inadequacies in homogeneity, ball-milling for producing coating layer is available for industrial manufacturing and is not detrimental to the host material. 41,42n this study, we offer an easy wet-chemical method based on TiF 4 for a composite coating of TiO 2 and LiF that modies the NCM622 surface.Aer annealing at a high temperature, a composite coating with a high degree of chemical stability was created on the NCM surface.TiO 2 coating can stabilize the structure on the outer layer of NCM622 to a certain degree.In order to shield the NCM structure, inhibit side reactions of the electrolyte and the electrode, and offer active sites for quick transfer of Li + and electrons, it is possible to use the TiO 2 and LiF for CEI formation.This overcomes the disadvantages of inconsistent distribution and excessive thickness.The electrochemical performances have been signicantly enhanced thanks to the successful in situ construction of a thin homogeneous TiO 2 -LiF layer on the surface of the host material through heat decomposition.At 2.7-4.3V at 1C, the electrochemical properties of NCM622 were evaluated, then the mechanism of improvement was investigated.

Preparation of TF-NCM622
2.1.1Synthesis of materials.Commercial NCM622 was bought from Canard, while supplies for TiF 4 (99%) powder were bought from Aladdin.To create a consistent solution with a certain concentration, TiF 4 is rst dissolved in anhydrous ethanol.Aer that, the appropriate quantity of NCM622 powder had been added to the solution in accordance with the predetermined weight ratio (TiF 4 : NCM622 = 0.2% or 1%).The mixture had a 30 minute ultrasonic treatment before being moved to an oil-bathing pot, immediately agitated at 60 °C until it got viscous, subsequently removed, evaporated in a vacuum oven for 10 hours, followed by calcined in the air at 400 °C for an hour to produce the nished product.The labels on the goods read NCM622/TF-0.2%and NCM622/TF-1%.

Electrode preparation and cell assembly
PVDF 5130, super P and NCM622 (1 : 1 : 8, wt%) are dissolved in N-methyl-2-pyrrolidone (NMP) to prepare the NCM622/TF electrode slurry.The NMP was removed by casting the resulting slurry on a foil of aluminum and drying it at 110 °C overnight under vacuum.The NCM622 weight loading was 3.4 mg cm −2 aer disc electrodes with a diameter of 10 mm were pierced.Lithium foil serving as anodes, a Celgard 2500 membrane serving as the separator, and ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) (v/v/v = 1 : 1 : 1) serving as the electrolyte.The electrolyte volume is 60 mL added to the coin cell.

Materials characterization
The sample's crystal phase was identied using a technique known as X-ray diffraction (XRD, Rigaku Ultima IV).The scanning electron microscopy technique (SEM, ZEISS MERLIN Compact) was used to analyze the sample's morphology, and a dispersive spectrometer with energy dispersive (EDS, FElinspect-F50) was used to identify the distribution of the elements.Using a TEM (Talos F200s), the shape and structure of the lattice of the coated layer were investigated.The composition of its surface was investigated using XPS (Thermo Kalpha, XPS), an X-ray photoelectron spectroscopy technique.

Electrochemical measurements
The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements performed on an CHI660E electrochemical workstation.Prior to the cycle, the EIS was assessed at 3.0 V of voltage, 10 −2 to 10 5 Hz of scanning frequency, and 5 mV of amplitude.

Results and discussion
Fig. 1a displays the XRD patterns for samples of coated and pure NCM622.Since there are no extra second-phase diffraction peaks in any of the samples, the coating concentration is quite low.The peak intensities of the NCM622/TF-0.2%and NCM622/ TF-1% samples were comparable to those of pure NCM622, but the (003) peak values of the NCM622/TF-0.2%and NCM622/TF-1% samples are poor, possibly as a result of the thick coating obstructing the equipment detection signal.The NCM622 surface was covered with a thin layer of TiF 4 using a wet chemical process, and during high-temperature solid-state sintering, TiF 4 gradually changed into TiO 2 .The impact of the composite barrier has been predicted theoretically as Li atoms presented on the NCM622 surfaces reacting with gaseous uoride compounds broken down in TiF4 to generate LiF.SEM was used to examine the morphology of samples following alteration (Fig. 1b).The elements Ti, F, O, Ni, Co, and Mn are evenly dispersed throughout NCM622 according to the EDS-mapping of NCM622/TF-0.2%.
The samples that were collected before and aer wrapping were subjected to XPS testing in order to further verify the presence and chemical makeup of the coating. 43,44The entire XPS spectrum (Fig. 2) shows peaks of Ti 2p, O 1s, and F 1s.The ndings demonstrate that F and Ti do not totally evaporate and vanish aer being calcined at 400 °C; rather, they continue to cling to NCM622's surface.In the Ti 2p spectra (Fig. 2a), the outer layer of the pure NCM622 sample did not exhibit any Ti signal, but the coated NCM622/TF-0.2%samples surface did exhibit the typical Ti 2p orbital peak spectrum.Both orbital peaks correspond to Ti 4+ in TiO 2 , and signal peaks of Ti 2p 3/2 and Ti 2p 1/2 were seen at 458.3 and 464.3 eV, respectively.The essential signal peak of NCM622 lattice oxygen is located at 529 eV in the O 1s spectrum (Fig. 2b).The oxygen vacancy on the surface of NCM622 is the cause of the oxygen adsorption peak at 531.3 eV.It can be noted that the NCM622/TF-0.2%sample's oxygen vacancy peaking/lattice oxygen peak intensity was weaker than the pure NCM622, indicating the number of oxygen vacancies in the topmost layer of the NCM622 aer its coating was reduced, which is advantageous to the device.The Ti-O bond in the TiO 2 compound is represented by the yellow peak at 529.7 eV, which shows that the TiF 4 was converted into the TiO 2 at a high temperature.The signal in the F 1s spectra that corresponds to the peak of elemental F was also exclusively seen on the surface of the NCM622/TF-0.2%sample (Fig. 2c).The peak at 685.0 eV is attributable to LiF produced on the NCM622 surface, further demonstrating that Li exposure on the surface layer during the high-temperature heat treatment reacted with the gaseous uorine compound dissolved by TiF 4 .The aforementioned ndings demonstrate that the TiO 2 -LiF coating applied to the NCM622 surface within this study was successfully created.
The coated NCM622 was characterized by HR-TEM techniques and EDX-mapping in order to further observe the structure of the microstructure and external element distribution.As can be seen in Fig. 3a and b, the coating of the NCM622 particles has been applied, and it is largely nished.The coating is distinct from the NCM622 structure.On NCM622/TF-0.2% and NCM622/TF-1%, the coating layer thickness is approximately 10 nm (Fig. 3a) and 43 nm (ESI, Fig. S1 †).While 45 nm will disrupt the electron and ion transport of NCM622 particles, which will impair the cycle stability and surface impedance, 10 nm of TiO 2 -LiF is a more acceptable coating layer thickness.The edge coating layer's crystal orientation was distinct from the lattice stripes of the NCM622 as can be shown in the high-resolution TEM pictures (Fig. 3b).In the electron diffraction (EDS) spectrum (Fig. 3d-h), in addition to the signals from the elements Ni, Co, Mn, and O present throughout the NCM622 material, signals from the elements F and Ti were also discovered and evenly dispersed onto the NCM622 surface, demonstrating the effectiveness of the experimental coating.
A charge-discharge measurement of the NCM622/Li half-cell had been carried out at 2.7-4.3V and 1C (1C = 180 mA h g −1 ) to conrm the impact of the TiO 2 -LiF layer on the electrochemical properties of NCM622.The results are given in Fig. 4a.At 0.2C, the initial coulombic efficiency (CE) of pure NCM622, NCM622/ TF-0.2%, and NCM622/TF-1% is 84.5%, 82.5%, and 80.71%, respectively.Since the materials' cycle curves are comparable, it can be assumed that the specic capacity is hardly affected by the TiO 2 -LiF coating.The discharge capacity value for both materials at different rates is shown in Fig. 4b.The discrepancy between the discharge specic capacities grows as the discharge current density rises.The capacities of the pure NCM622, NCM622/TF-0.2%,and NCM622/TF-1% were 64.7, 87.5, and 155.7 mA h g −1 , respectively, as the current density increased to 5C, with the gap continuously widening.Due to the optimized lithium-ion migration circumstances, which boosted the lithium-ion transfer rate and encouraged the liberation of its capacity, the original capacity of coated NCM622 was not  A crucial element of CEI, the LiF that forms on the outer layer of NCM622 aer coating can prevent hazardous negative interactions between NCM622 and the electrolyte, lower the increase in charge transfer impedance, lower the loss of the chemically active lithium, and increase capacity of NCM622.
To study the kinetics of the coated NCM 622 cathode material, EIS was recorded on pure NCM622 and NCM622/TF-0.2%before cycling, aer 20 and 50 cycles, respectively.The model in   impeding redox interactions between NCM622 and the electrolyte.The pure NCM622 sample's retention rate was only 68.9% (111.0 mA h g −1 ), the NCM622/TF-0.2%sample's capacity retention rate was 79.7% (129.4 mA h g −1 ), indicating that the TiO 2 -LiF hybrid coating still had an effective protective function on the NCM622.

Conclusion
In this study, a TiO 2 -LiF composite coating was created by heating NCM622 in one step with TiF 4 .The coated NCM622 demonstrated outstanding cycling performance.The NCM622/ TF-0.2% exhibited excellent electrochemical performance, and demonstrated an retained capacity rate of 79.7% aer 200 cycles in a voltage range of 2.7-4.3V at 1C, pointing to a potential for future commercial application.This work demonstrates that the TiO 2 -LiF coating effectively prevents corrosion of the NCM622 bulk components by damaging chemicals in an electrolyte, strengthens the surface interior structure of the NCM622, and signicantly alleviate the structural deterioration of the NCM622.

Fig. 5
Fig. 5 was used to t the data.The resistance of CEI lm (R f ) and electron transfer impedance (R ct ) is shown in the Nyquist plots.R ct denotes the interfacial reactions for Li + migration on the NCM622 surface.The R s impedance values of pure NCM622 and NCM622/TF-0.2%have little change in different cycles.Aer 20 and 50 cycles, R f decreased from 49.5 U to 37 U and from 56.1 U to 40.6 U, respectively, indicating that the coating improved the formation of CEI lm.The R ct of pure NCM622 becme larger at different cycles, which suggests that product of the side reation greatly obstructs Li + transfer.While for NCM622/TF-0.2%, the Rct decreased aer different cycles, it exhibits that the TiO 2 -LiF coating layer prevents HF corrosion of the electrode while